The present invention relates to a control system of an infinitely variable belt-drive automatic transmission for a motor vehicle, and particularly to a hydraulic control system for an infinitely variable transmission.
U.S. Pat. No. 4,369,675 discloses a control system for an infinitely variable belt-drive transmission. The transmission comprises an endless belt running over a drive pulley and a driven pulley, each pulley is so designed that the running diameter of the driving belt on the pulleys varies by a hydraulic control system.
In the hydraulic control system, a line pressure of oil is continuously applied to a servo chamber for the driven pulley and further the line pressure is applied to another servo chamber for the drive pulley by a transmission ratio control valve after starting the operation so as to control to upshift the transmission. In the conventional system, before starting the transmission operation, the servo chamber of the drive pulley is communicated with a drain port of the transmission ratio control valve to drain the servo chamber, so as to provide a low speed stage at a maximum transmission ratio.
However, if the servo chamber of the drive pulley is kept draining before the starting of the transmission operation, all or a part of the oil in the servo chamber is discharged, admitting air in the servo chamber. Accordingly, when the line pressure is applied to the servo chamber of the drive pulley through the transmission ratio control valve, the air admitted in the servo chamber is compressed first, and then the pressure of oil elevates to start the upshifting of the transmission, resulting in delays in elevation of the oil pressure and in response of transmission operation. Further, when an accelerator pedal of the vehicle is fully depressed, the upshifting of the transmission starts after the speed of an engine reaches a high value. Accordingly, the drive pulley rotates at high speed under the condition of a small amount of oil in the servo chamber until the upshifting starts, which will cause troubles in sliding portions and sealing parts in the servo device, because of a small amount of oil in the servo chamber.